granting send on behalf of permissions in exchange management shell

Set full access permissions
Add-MailboxPermission mymailbox@server.com -User grantmerights@server.com -AccessRights FullAccess -InheritanceType All

Send on behalf of permission (give dlopez send of behalf of for rclark)
Set-Mailbox mymailbox@server.com -GrantSendOnBehalfTo grantmerights@server.com


check settings
Get-MailboxPermission -Identity mymailbox@server.com -User grantmerights@server.com
get-mailbox -Identity mymailbox@server.com | fl *grantsendonbehalfto